1. Field of the Invention
The present invention relates to a method for detecting a non-superconducting transition of a superconducting wire.
2. Description of Related Art
Superconducting wires are applicable to, for example, magnetic resonance imaging devices, superconductive magnets of magnetic levitation railways, magnetic bearings, electric motors and the like, and superconductive cables, and toward such practical use, research for ensuring the reliability during operation of the superconducting wires are being actively conducted.
Since the critical temperature (the temperature of the upper limit indicating superconductivity) is generally lower than an ordinary temperature, a superconductor that constitutes a superconducting wire is used by being cooled to the critical temperature or less using a cooling medium such as liquid helium or liquid nitrogen, or a refrigerator. However, even if a superconducting wire is cooled to the critical temperature or less from the outside, a non-superconducting transition, that is, the transition from the superconducting state to the non-superconducting state would occur due to a thermal disturbance in a predetermined portion of the superconducting wire in some conditions. In this case, the temperature of the superconducting wire increases due to an occurrence of Joule heat, and it accelerates the non-superconducting transition of the surrounding area, leading to the problem of an expansion of the region in a non-superconducting state (quench phenomenon).
Japanese Patent No. 2577682 discloses a method for detecting minute temperature increase from the voltage of a carbon film that is provided on a superconductor, in order to detect slight temperature increase immediately prior to quenching in which the superconductor transitions to a non-superconducting state due to a thermal disturbance. However, the method disclosed in Japanese Patent No. 2577682 utilizes the property of the electrical resistance value being remarkably large with respect to the temperature of the carbon film, in an extremely low-temperature region of several K (Kelvin) from the temperature of liquid helium (refer to FIG. 7 in Japanese Patent No. 2577682). For that reason, it is difficult to apply the method to a high-temperature superconductor in which the critical temperature is 77 K or more (for example, approximately 100 K).
Japanese Unexamined Patent Application, First Publication No. H08-304271 discloses a method for detecting quench of a superconductor that makes polarized light from a light source enters an optical fiber wound around a superconducting wire, detects a phase difference of the polarized light from the optical fiber, and detects an unusual polarization state of the light that has transmitted through the optical fiber.
Also, Japanese Unexamined Patent Application, First Publication No. H07-170721 (particularly in the fourth invention thereof) discloses a method for detecting quench of a superconducting wire in which an optical fiber is attached to the outside of a superconducting wire, and an abnormality of the superconducting wire is detected by measuring the reflected light from the deformation portion of the optical fiber due to mechanical deformation of an unusual portion in the superconducting wire during electrification or by measuring the transmitted light from the other end of the optical fiber.
However, the methods disclosed in Japanese Unexamined Patent Application, First Publication No. H08-304271 and Japanese Unexamined Patent Application, First Publication No. H07-170721 can only determine the presence of an abnormality in an optical fiber by an increase in positional shifting or deformation of an optical fiber due to moving the superconducting wire caused by the quenching. And by the methods, it is not possible to measure the temperature variation in detail.
U.S. Pat. No. 6,072,922 and Wolfgang Ecke et al., “Fiber optical grating sensors for structural health monitoring at cryogenic temperatures”, Proceedings of SPIE, Vol. 6530, 653002 (2007) disclose a method for measuring temperature under an extremely low temperature with an optical fiber temperature sensor that uses a fiber Bragg grating (FBG). A FBG is an optical fiber device in which a periodical refractive index modulation (grating) is formed in the core of an optical fiber, and it has a selective reflection property of a specified wavelength (Bragg wavelength) that is determined by the refractive index of the core and the grating period.
In U.S. Pat. No. 6,072,922, a coating material (a coating) such as aluminum (Al) or polymethyl methacrylate (PMMA) with a larger thermal expansion coefficient (TEC) than silica, which is the main component of an optical fiber, is provided around the FBG portion of an optical fiber, to enhance the sensitivity of the temperature sensor by increasing the Bragg wavelength shift due to temperature. Also, a measurement example of strain, temperature, and linear expansion is shown in Non-patent Document 1.
U.S. Pat. No. 6,072,922 and Wolfgang Ecke et al., “Fiber optical grating sensors for structural health monitoring at cryogenic temperatures”, Proceedings of SPIE, Vol. 6530, 653002 (2007) only disclose being able to measure the temperature of a medium in the case of using a FBG provided with a coating material, for example, a FBG provided with a coating material being immersed in a uniform medium as shown in FIG. 21 of U.S. Pat. No. 6,072,922.
Also, in the case of using the FBG disclosed in U.S. Pat. No. 6,072,922 for temperature measurement of a high-temperature superconductor, since a high-temperature superconductor in the shape of a wire is lacking in a deformation property, it is not possible to place the superconducting wire completely around the wire. That is to say, the medium around an FBG provided with a coating material cannot be made homogeneous. Also, in consideration of the thermal conductivity from the wire, even if the coating material around an optical fiber is firmly attached to a superconducting wire, since difference of the thermal expansion coefficient between the coating material and the superconducting wire, the expansion/contraction of the coating material is restricted. For this reason, there is a risk of having an adverse influence on accuracy and response speed of temperature measurement.
The present invention has been achieved in view of the above circumstances, and has an object to provide a method for detecting the non-superconducting transition of a superconducting wire that, in addition to detecting the temperature variation accompanying the non-superconducting transition with high accuracy and responsiveness, can detect with greater precision the state of a superconducting wire in which the non-superconducting transition has occurred based on a temperature variation of the superconducting wire.